Most studies on gaming addiction have targeted adolescents in schools, while studies on gaming addiction among out-of-school adolescents remain scarce. Therefore, this study investigated the influence of parental and peer attachment on gaming addiction, mediated by social stigma among Korean out-of-school adolescents. The Dropout Youth Panel Survey was used for a sample of 437 dropout adolescents. The results showed that out-of-school adolescents' social stigma fully mediated the relationship between parental and peer attachment and gaming addiction. Parental attachment significantly predicted out-of-school adolescents' gaming addiction by decreasing their social stigma. Peer attachment also influenced gaming addiction in out-of-school adolescents via the mediated effects of social stigma. No direct effect of parental and peer attachment on gaming addiction was found. Social stigma played an important role in decreasing levels of gaming addiction. In addition, our study revealed that the direct and total effects of parental attachment on gaming addiction were greater than the effect of peer attachment. This study empirically verified the importance of social stigma based on modified labeling theory and provides a valid mechanism to describe how Korean out-of-school adolescents develop gaming addictions. The findings suggest effective strategies for prevention and treatment for dropout adolescents in South Korea.